This invention relates generally to fuel cells, and more particularly to a system and method for purging a fuel cell system with coolant.
Fuel cell systems are electrochemical power sources for both stationary and mobile applications. One type of fuel cell employing a solid polymer electrolyte membrane (PEM) has shown particular promise as an automotive power source. That type of fuel cell includes a membrane/ electrode assembly (MEA), with fuel, oxidant and coolant flow fields adjacent to the anode and cathode electrodes. The PEM fuel cells operate at relatively low temperatures, thus facilitating their application to automotive uses. Fuel cells do, however, have a characteristic that may present a drawback, particularly in automotive applications which typically require frequent shutdown and startup of the fuel cell.
Unless the fuel cell is in some way purged or otherwise inerted when it is being shutdown and started up, undesirable processes and reactions may occur, or continue to occur. If the MEA of the fuel cell is not protected, as by inerting the anode and/or cathode flow fields by flooding with a purge gas, the chemical processes may occur in some form, and lead to undesirable results. Normally it is intended for the output voltage from the fuel cell to be reduced or terminated at shutdown, however the failure to inert may allow the process to continue in a way that depletes fuel volume on the anode side and creates a vacuum. If the system is not sufficiently leak tight, air may be drawn into the fuel side and lead to unwanted results. Similar concerns exist during start-up in order to avoid gas interfaces which may give rise to unwanted reactions at that time.
It has been observed that the failure to adequately inert a fuel cell system may allow a fuel/oxidant (air) interface to exist, even if temporarily, at regions in the fuel cell, and such interface may lead to serious safety concerns, as well as degradation in the performance and life of the cell.
Although the desirability of inerting fuel cells is well known and has typically been accomplished through the use of an inert purge gas such as nitrogen, that particular process may detract from the overall economics and efficiency of fuel cell use. More particularly, the need to obtain, transport and/or frequently refill a source of inerting gas such as nitrogen is a significant obstacle to the acceptance and use of fuel cells as an automotive power source. The use of an inert gas bottle adds undesirable weight, volume and complexity, and should be avoided if possible. To mitigate or avoid this obstacle, other techniques have been advanced as alternatives to the use of inert purge gas to inert fuel cells.
In one alternative, described in U.S. patent application Ser. No. 09/133,768, filed Aug. 8, 1998 for xe2x80x9cSelf-Inerting Fuel Cell Systemxe2x80x9d, and assigned to the assignee of the present invention, fuel, oxidant and coolant are individually controlled and caused to flow through respective flow fields in fine pore plates adjacent the anode and cathode sides of a membrane/electrode assembly in a PEM type fuel cell. Through selective control of the relative pressures of the fluids in the respective fuel, oxidant and coolant flow fields, coolant is permitted, at shutdown, to migrate through the fine pore plates and flood the fuel and oxidant (collectively xe2x80x9csreactantsxe2x80x9d) flow fields, thereby displacing the reactants and inerting the fuel cell system.
While the aforementioned Self-Inerting system of U.S. Ser. No. 09/133,768 does provide a means of inerting a fuel cell without requiring the cost and inconvenience of a nitrogen purge, it relies solely on the mechanism of the coolant migrating through the fine pore plates and displacing (i.e., purging) the reactants. Although such mechanism may be sufficient in many instances, in other instances it may be preferable to utilize the coolant as a purge or inerting agent in a more conventional manner, either alternatively or supplementally. The foregoing needs and preferences exist not only when shutting down the fuel cell, but also during start-up.
In view of the foregoing, it is an object of the present invention to provide a fuel cell inerting system which overcomes the limitations and disadvantages of prior fuel cell systems, and particularly prior inerting/purging systems for fuel cells. Other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.
The present invention is a system for protecting fuel cells, especially of the PEM type, particularly during transient periods such as start-up and shut down.
Accordingly, the present invention relates to an inerting system, both method and apparatus, for a fuel cell stack of the type that has a membrane/electrode assembly (MEA), which MEA includes anode and cathode electrode catalyst layers on respective opposite sides of the membrane, and wherein the fuel cell stack has a fuel reactant flow path on the anode side of the MEA, an oxidant reactant flow path on the cathode side of the MEA, and a coolant flow path in fluid communication with the anode and the cathode sides of the MEA, and further wherein the fuel reactant flow path, the oxidant reactant flow path and the coolant flow path each have a respective inlet to and a respective outlet from the fuel cell stack, and wherein a fuel reactant supply, an oxidant supply and a coolant supply are applied to the fuel reactant inlet, the oxidant reactant inlet and the coolant inlet during normal on load operation of the stack.
More specifically, the invention relates to the method and apparatus for purging such fuel cell stack with coolant, typically water, during the transient operations of start-up and shutdown. Appropriate means and processes are provided to redirect the supply of coolant from the coolant flow path inlet to one, the other, or both of the inlets to the fuel reactant and oxidant reactant flow paths. Similarly, feed of the supplies of fuel and/or oxidant to those respective inlets is terminated or provided in a controlled sequence. The admission of coolant to either, or both, of the fuel reactant or oxidant reactant flow paths serves to displace the respective reactant and to inert the respective anode and/or cathode when the reactant has been completely displaced. Similarly, reintroduction of the respective reactants displaces the coolant and returns the system to a condition for on load operation. Sensor(s) at the outlets of the respective reactant flow paths indicate when coolant has displaced the respective reactant, to thereby signal or control completion of one operation and initiation of another.
For shutdown, one, or both, of the fuel and oxidant reactant flow paths may be purged with system coolant in the foregoing manner. Similarly, for start-up, one, or both, of the fuel and oxidant reactant flow paths is purged with system coolant to inert the respective anode and/or cathode during that phase.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.